President Donald Trump’s lawyer says the criminal investigation into possible collusion with Russia in last year’s election could be over by December, but Special Counsel Robert Mueller’s probe is expected to continue well into next year, according to a U.S. official.

Mueller continues to gather evidence and pursue investigative leads, as shown by steps like a subpoena he sent to more than a dozen Trump campaign operatives in October, according to the official with knowledge of the investigation, who requested anonymity to speak about sensitive matters.

Ty Cobb, the top White House lawyer handling the probe, has been consistently optimistic about Mueller’s probe and its likely outcome, predicting the investigative cloud hanging over Trump and the White House should clear by early next year. “The office of special counsel is working diligently to complete its interviews” and the White House has been cooperating with the investigation to expedite its conclusion, Cobb said in an interview.

But the official with knowledge of the investigation, as well as outside legal experts, made clear that months of work still lie ahead for Mueller. For one thing, Mueller indicted Trump’s former campaign chairman, Paul Manafort, last month, as well as another campaign aide, Rick Gates, for money laundering and other crimes. Manafort and Gates have said they aren’t guilty, and Mueller’s litigation against them is expected to continue well into 2018, the official said.

Mueller was given a broad mandate when he was appointed by the Justice Department in May to investigate whether Trump or any of his associates colluded with Russia as well as any other matters arising from that inquiry.

To build his case, Mueller has had to pursue multiple investigate angles beyond the White House, a second U.S. official said. Those include potential obstruction of justice related to Trump’s firing of FBI Director James Comey, financial dealings in the U.S. and abroad by Trump family members and associates, and Moscow’s efforts to manipulate Facebook and other social-media platforms, the official said.

“This investigation will continue through 2018,” said Jeffery Cramer, a former federal prosecutor who is now managing director for Berkeley Research Group.

“It seems like the White House is setting up a straw man and groundless expectations,” Cramer said. “The only running clock is the statute of limitations on any potential charges.”

Cobb has said he expects interviews with White House staff to wrap up shortly after Thanksgiving and that the vast majority of documents requested from the White House by Mueller were handed over last month.

The first official said it’s possible that Mueller’s team of more than two dozen prosecutors and FBI agents will complete an opening round of interviews with key Trump aides who worked in the White House by the end of the year, but additional interviews could be scheduled later.

Among those who have been interviewed are former Chief of Staff Reince Priebus, former spokesman Sean Spicer and National Security Council chief of staff Keith Kellogg, according to people familiar with the investigation. Mueller has also indicated he wants to speak with White House Counsel Don McGahn and communications director Hope Hicks, said another person close to the inquiry.

The recent subpoena was intended to ensure that Mueller receives all the documents he’s seeking, the first official said. Mueller’s next step is to review the materials to determine whether additional subpoenas are needed or new lines of investigation need to be opened, the official said.

The indictment against Manafort and Gates demonstrates that Mueller is methodically building cases that take time, said the second U.S. official, who also asked to remain anonymous.

Others whose activity is under investigation include Trump’s former national security adviser Michael Flynn, Trump’s son Donald Trump Jr. and Trump’s son-in-law Jared Kushner, the second official said.

Mueller also revealed last month that he secured a cooperating witness — George Papadopoulos, a junior foreign-policy adviser to the Trump campaign who pleaded guilty to lying to the FBI about the timing of his contacts with Russian operatives.

“Did Papadopoulos record any conversations after he pleaded guilty?” Cramer, the former prosecutor, asked. “Will Manafort cooperate to spare himself some potential prison time?”

Mueller has staffed his team “with some of the best investigators, former prosecutors, and an individual from the solicitor general’s office who has argued more Supreme Court cases than most anyone,” Cramer said. “This team was not established to take an easy plea on lying to the FBI and Manafort’s money laundering, tax evasion, and lack of proper filings.”

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